Shotgun Bores Are Smooth

Why are some shotguns called smoothbores? The Rifle,
Handgun, and Shotgun Bores
drawing shows the inside
of a shotgun barrel, which
is smooth. Therefore, some
shotguns are called smoothbores.
Also, the shotgun barrel
is thinner than a rifle
barrel. The barrel looks
like a piece
of thin steel pipe. Shotguns
usually shoot pellets,
also called shot.